Generally, an OTFT, which is a device for driving a next-generation display, is under active study. The OTFT includes, as a semiconductor layer, an organic film in lieu of a silicon film, and is classified into a small molecular OTFT such as oligothiophene, pentacene and so on, and into a polymer OTFT such as polythiophene and so on, depending on the type of material of the organic film.
An organic electroluminescent display using the OTFT as a switching device includes a plurality of pixels arranged in the form of a matrix on a substrate, each pixel being provided with an organic electroluminescent device including two OTFTs, for example, a switching OTFT and a driving OTFT, a capacitor, upper and lower electrodes, and an organic film disposed between the upper and lower electrodes.
Typically, a flexible organic electroluminescent display includes a flexible substrate as a substrate. The flexible substrate includes a plastic substrate. The plastic substrate has very poor thermal stability and thus requires a low-temperature process to manufacture an organic electroluminescent display.
Hence, because the OTFT using the organic semiconductor thin film as the semiconductor layer may be subjected to a low-temperature process, it is receiving attention as a switching device for a flexible organic electroluminescent display.
However, materials usable for the organic semiconductor layer are problematic in that they are relatively expensive. For this reason, the demand for methods of reducing the manufacturing cost of the OTFT by using the expensive organic semiconductor in a small amount continues.
In the case where the organic semiconductor material is exposed to the surface, the properties thereof may change due to environmental causes, and thus there is an additional need for a protective layer for protecting it. When the protective layer is formed on the organic semiconductor layer through a solution process, problems of damage to already-made underlayers due to the solvent used in subsequent processes may occur. In order to more effectively achieve the solution process, research into fabrication of a multilayered thin film through a single coating process is being conducted.
Thus, attempts to blend the organic semiconductor polymer with the insulating polymer so as to combine the electrical properties of the organic semiconductor, the mechanical properties of the insulating polymer, and the material properties of the inexpensive material are being made.
Recently, H. Sirringhaus Group, UK, has succeeded in the manufacture of a device able to maintain charge mobility even when poly-3-hexyl thiophene (P3HT) is used only in an amount of about 3 wt % upon blending of P3HT which is a type of organic semiconductor with polystyrene (PS) or polyethylene (PE) which is an insulting polymer, and the results thereof are reported in Nature Materials, 5, 956 (2006) and disclosed in PCT patent WO 2008/001123 A1. However, in the case of this method, the device able to maintain charge mobility despite the use of a small amount of P3HT may be manufactured only under a condition in which the insulating polymer used is a crystalline polymer, for example, isotactic-PS or high-density PE. The reason is that P3HT is spread on the substrate while being crystallized, and the insulating polymer is then formed on the P3HT layer, thus obtaining a structure in which the P3HT layer and the insulating polymer layer are vertically phase-separated on the substrate. Accordingly, even when P3HT is used in a small amount, a charge transfer passage may be formed between source and drain electrodes. However, this method is difficult to commercialize because of a complicated manufacturing process including the crystallization of P3HT and then the solidification of the insulating polymer. Further, because a drop-casting process is adopted, it makes it difficult to manufacture a device through application of a uniform film on a large area.
Using a blend of P3HT and polymethylmethacrylate (PMMA), P3HT and PMMA are vertically phase-separated on the substrate so that the PMMA layer is located on the P3HT layer to thus use the PMMA layer as the protective layer of the P3HT layer, which was studied by A. Arias in Palo Alto Research Center (Adv. Mater. 19, 2900 (2006)). However, PMMA has a limitation in that it is only used as the protective layer, and also, P3HT should be added up to 40% and is thus unfavorable in terms of reducing the use of the organic semiconductor.
The present research group has recently studied use of PMMA as the dielectric layer of the OTFT through vertical phase separation of a blend of P3HT and PMMA such that PMMA is located at a lower position and P3HT is located at an upper position on the substrate (Adv. Mater, 20, 1141 (2008)). In this case, even when 3 wt % of P3HT is used, the OTFT has performance equal to that of an OTFT manufactured using 100% P3HT. Because the manufacturing process is performed through spin casting, commercialization is possible compared to research results of H. Sirringhaus Group.
According to all of the aforementioned results, the charge transfer passage may be maintained in a lateral direction even when the organic semiconductor is added in a small amount to the blend of organic semiconductor and insulating polymer due to vertical phase separation on the substrate. However, because various factors (solvent evaporation rate, surface energy of substrate, concentration of solution, etc.) are involved in the vertical phase separation, many limitations are imposed on the vertical phase separation on the substrate. Also, a horizontal phase separation force coexists with a vertical phase separation force in the polymer blend, and thus it is difficult to uniformly achieve the vertical phase separation on a large area.
Therefore, methods of easily manufacturing a thin film of a blend of organic semiconductor and insulating polymer having superior electrical and physical properties even with the use of a small amount of organic semiconductor are in constant demand.
Hence, the present invention is intended to provide a method of manufacturing a novel functional thin film in which organic semiconductor nanofibrils are dispersed in the form of a network in the insulating polymer layer using a blend of organic semiconductor/insulating polymer, and fabrication of an OTFT having superior performance (i.e., improved environmental stability) even when using a small amount of organic semiconductor.